1. Field of the Invention
The present invention relates to an image forming apparatus.
2. Description of the Related Art
Conventional printers, copiers, facsimile machines, and other such image forming apparatus, specifically printers, for example, are adapted to form images on the basis of parameters specifying various image forming conditions. These parameters are normally optimized by modifying preset base values according to the number of pages that have been printed, the type of paper being used as the printing medium, environmental factors, and so on. The base values and correction formulas are derived empirically, by experiments conducted by the manufacturer.
Image density is a factor that particularly needs to be regulated because of its strong effect on image quality. In Japanese Patent Application Publication No. 2001-051459, Endo discloses an image formation device that transfers certain patterns onto an intermediate transfer body, uses a density sensor to measure the density of the transferred patterns, detects aging changes in the image formation device on the basis of the measured image density, and makes compensating adjustments that maintain a uniform image density.
The density sensor employed by Endo, however, only obtains the average density of the measurable image area, so while it can be used to improve average overall image density, it cannot accurately measure image density at the scale of individual dots. Image density at this scale becomes important when a printer is used to print barcodes, watermarks, and other patterns consisting of extremely fine lines or ultra-small dots that must be reproduced accurately on a scale of a few tens of micrometers (a few hundredths of a millimeter).
A direct way of meeting this requirement would be to print a test image including fine lines and ultra-small dots, use a special instrument with microscopic optical resolution to scan the printed image and measure the dimensions or of these features, and adjust the printing parameters until the desired dimensions are obtained, but this would be a troublesome procedure entailing separate scanning, measurement, and adjustment processes that could not easily be automated, and would require expensive extra equipment not normally provided as part of a printer.
The problem becomes particularly difficult when the barcode, watermark, or other pattern is printed with an invisible toner or ink and embedded in a visible image on the same sheet of paper. Such invisible patterns are read by special scanners sensitive to a restricted range of wavelengths of light. Accurate adjustment by the above direct method would require the use of a special measuring instrument with matching spectral sensitivity, and it would also be necessary to contend with the presence of colorant in the invisible printing agent. If the dot size of the invisible pattern is not accurately adjusted, the embedded pattern may fail to perform its function because the intended scanner cannot recognize the pattern.